Open cmd and type echo %QT_PLUGIN_PATH% if it's not empty then delete it (just google: delete environmental variable windows #, where # is your windows version)
Open cmd and type echo %PATH% and press enter. You'll see a list separated by ; of folder paths. Go line by line and make sure they do not point to a folder that contains a platforms folder that has qwindows.dll in it. If you find such a path edit the PATH variable and remove it (just google: edit environmental variable windows #, where # is your windows version)